Earliest life on Earth: Evidence from the Barberton Greenstone Belt, South Africa

Abstract

Studies investigating the structure and diversity of Earth's record of life older than 3.2 Ga are restricted to two locations worldwide in which sedimentary rocks have escaped regional high-grade metamorphism and penetrative deformation: the Pilbara Craton of Western Australia and the Barberton Greenstone Belt in the Kaapvaal Craton of South Africa. This paper provides a South African perspective on the evidence of Paleoarchean life; a record that is often overlooked in the literature. It aims to summarize and critically review previously reported claims of early life in the BGB, gives an overview of the latest findings, and provides an outlook on potential future discoveries.The ~15 km thick, volcanic-sedimentary succession making up the Barberton Supergroup was deposited between 3.55 to ca. 3.20 Ga and can be subdivided in three stratigraphic units that provide a unique window into a diverse and widespread Paleoarchean microbial ecosystem landscape. Putative biosignatures occur almost throughout the entire BGB stratigraphy and range from carbonaceous cherts containing filamentous, spheroidal, and lenticular microstructures, traces of hydrothermal biofilms, photosynthetic microbial mats, remnants of pseudocolumnar stromatolites, and large, organic-walled spheroidal microfossils of currently unknown affinity. The BGB also contains one of the world's oldest known record of tufted microbial mats, which extensively colonized tidally-influenced, siliciclastic shorelines and were most likely formed by filamentous photosynthesizers. Other mat-associated biosignatures include silicified gas bubbles, domes and lenses that likely formed due to metabolic activity or the decay of buried organic matter. Some of these subsurface voids beneath the cohesive mats were inhabited by the earliest known forms of cavity-dwelling microbial communities that were probably dominated by chemotrophic or photosynthetic microbes. Recently discovered terrestrial microbial mats, once thriving in a fluvial-dominated setting, represent the oldest macroscopically-visible fossil traces of life on land, which is also supported by the occurrence of nearby paleosols that carry signals of biogenic sulfur fractionation.The wealth of preserved microbial biosignatures from marine, fluvial, hydrothermal, and possibly planktonic settings combined with the high spatial and temporal resolution of the Barberton Greenstone Belt deposits is truly exceptional, Consequently, the BGB deserves an equal level of attention and protection for future generations like its Australian counterpart.